Kaori Yuse

Self-powered Wireless Health Monitoring Supplied by Synchronized Switch Harvesting (SSH) Method

The development of autonomous wireless sensors and actuators in order to design advanced structural health monitoring (SHM) systems is an exciting challenge for both the industrial and the academic communities. Some studies have dealt with the implementation of wireless devices. Almost all the studies about so-called autonomous systems today still require power supply. It means that a fully self-powered system has not been achieved yet. The present study shows the design of self-powered wireless health monitoring system, for which the energy is supplied by the original method called the synchronized switch harvesting (SSH) method. The piezoelectric elements and electrical circuit, located on the vibrating structure, harvest electrical energy from the direct conversion of mechanical energy vibration. Lamb wave transmission is chosen for the SHM of a composite beam of length around 30 cm. Some piezoelectric elements, used for energy harvesting and signal transmission, are located on the beam. The energy required to wake-up the micro-controller and to achieve two complete transmission cycles is only 1.5mJ. This small amount of energy can be harvested in a short time period for reasonable beam displacement levels. The study details the different trade-off concerning such a self-powered health monitoring device.

Evaluation by fast Fourier transforms analysis of energy harvesting in electrostrictive polymers driven by an electric field and a mechanical excitation

Electrostrictive polymers offer the promise of energy harvesting with few moving parts where power can be produced simply by stretching and contracting a relatively low-cost rubbery material. The use of such polymers for energy harvesting is a growing field, which has great potential from an energy density viewpoint. Basically, the relative energy gain depends on the current induced by the mechanical strain and frequency. A previous study in the Laboratoire de Génie Electrique et Ferroélectricité laboratory has indicated that one can measure the dielectric constant, the Young’s modulus, and the electrostrictive coefficient of a polymer film by determining the current flowing through the sample when the polymer film was simultaneously driven by an electrical field and mechanical excitation. The goal of this study has thus been to develop a solution for artificially increasing the coupling factor of electrostrictive materials, based on the optimization of the frequency of the electric field and the amplitude strain of the mechanical excitation leading to an increase in the generated current. When relating this parameter with a transverse strain of 5% and a bias field of 10 V/µm, it was found that such a process rendered it able to increase the converted power to 14 µW at a mechanical frequency of 6 Hz. The converted power was much higher than for the frequency of 3 Hz for which a low power was consumed by the polarization of the polymer. The theoretical analysis was supported by the experimental investigations. The contribution of this study provides a framework for developing energy harvesting techniques that should improve the overall performance of the system.

Elastocaloric effect in poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer

The elastocaloric properties of poly (vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] terpolymer were directly characterized using an infrared imaging camera. At a strain of 12%, a reversible adiabatic temperature variation of 2.15 °C was measured, corresponding to an isothermal entropy variation of 21.5 kJ m−3 K−1 or 11 J kg−1 K−1. In comparison with other elastocaloric materials, P(VDF-TrFE-CTFE) appears to represent a trade-off between the large required stresses in shape memory alloys and the large required strains in natural rubber. The internal energy of the P(VDF-TrFE-CTFE) polymer was found to be independent of the strain, resulting in complete conversion of the mechanical work into heat, as for pure elastomeric materials. The elastocaloric effect therefore originates from a pure entropic elasticity, which is likely to be related to the amorphous phase of the polymer only.

Stress/electrical scaling in ferroelectrics

The present paper introduces a scaling law correlating three factors of ferroelectric materials in quite simple functions: the polarization (P), the electrical field (E), and the stress (T). The law is based on the physical symmetries of the problem and renders it possible to express the mechanical stress (T) as an electric field equivalent {Eeq=Th[P(E=0,T)]} and, as a consequence, also the relationship between strain (S) and polarization (P). Three materials with various phase structure (tetragonal, rhombohedral, and morphotropic) were used for the verification. It was found that such an approach permitted the prediction of the maximal stress using only purely electrical measurements [i.e., measurements of S(E) and P(E)]. Once this law was validated for the compressive stresses, the mapping could be extended in order to predict the polarization behavior in the tensile stress zone. It was shown that the polarization behaved differently as a function of the compressive and tensile stresses. The scaling law...

Controllable Electrostriction of Polyurethane Film

Although their experimental errors can be observed, pure polyurethane (PU) elastomers are one of the most important class of polymers due to some remarkable electromechanical characteristics such as large electric field induced strain, high specific energy and fast speed of response. In order to obtain the large strain at low electric field, a dependence of the solidification condition on strain was investigated for pure polyurethane films. Optimum solidification condition to get thin film with 19 μm thickness remarkably enhanced the strain at high electric field at high electric filed, although they show the low strain at low electric field at low electric filed. The starting point of the convergence occurred at a lower electric field for the solidification condition to get thick film with 150 μm thickness as opposed to for the optimum condition to obtain the thin film with 19 μm thickness. Based on results of crystalline volume fraction and crystalline periodicity, strongly attributed to not only polarization, but also electrostriction, the strain was controlled by the solidification condition. The optimum solidified samples do not have convergence until 20 MV/m. Based on the prediction and experimental results, the electrostriction of PU films depended on its solidification condition.

Thickness effects in electroactive polymers actuators: a simple explanation and modeling

For practical use, the electrical field requirements of Electro Active Polymer (EAP) actuators have to be lowered down. Recently, we developed nano carbon filled polymeric films which can generate a large strain (30-50%) at moderate electrical field (less than 20 MV/m). Herein, the electrostrictive strain saturates versus electrical field and that the maximum strain depends strongly on the sample thickness. Combining polarization saturation effect and heterogeneities in the polymer thickness lead to a model that describes correctly the strain behavior versus electrical field, polymer thickness and frequency. A three-layer model was established which assumes that the polymer is not homogeneous along the thickness. Two outer and one inner layers exist, which must be formed during the polymer curing. It is considered that these layers have slightly different characteristics, such as permittivity. When the electrical field is input parallel to the polymer thickness, a different strain would take place in each layer according to the field distribution. Since the layers are attached together, the strain must be the same in each layer. Consequently stresses appear in the different layers. Introducing in this model a saturation of the polarization for high field leads to simulation results that fit well the experimental data.

Ultrasonic wireless health monitoring

The integration of autonomous wireless elements in health monitoring network increases the reliability by suppressing power supplies and data transmission wiring. Micro-power piezoelectric generators are an attractive alternative to primary batteries which are limited by a finite amount of energy, a limited capacity retention and a short shelf life (few years). Our goal is to implement such an energy harvesting system for powering a single AWT (Autonomous Wireless Transmitter) using our SSH (Synchronized Switch Harvesting) method. Based on a non linear process of the piezoelement voltage, this SSH method optimizes the energy extraction from the mechanical vibrations. This AWT has two main functions : The generation of an identifier code by RF transmission to the central receiver and the Lamb wave generation for the health monitoring of the host structure. A damage index is derived from the variation between the transmitted wave spectrum and a reference spectrum. The same piezoelements are used for the energy harvesting function and the Lamb wave generation, thus reducing mass and cost. A micro-controller drives the energy balance and synchronizes the functions. Such an autonomous transmitter has been evaluated on a 300x50x2 mm3 composite cantilever beam. Four 33x11x0.3 mm3 piezoelements are used for the energy harvesting and for the wave lamb generation. A piezoelectric sensor is placed at the free end of the beam to track the transmitted Lamb wave. In this configuration, the needed energy for the RF emission is 0.1 mJ for a 1 byte-information and the Lamb wave emission requires less than 0.1mJ. The AWT can harvested an energy quantity of approximately 20 mJ (for a 1.5 Mpa lateral stress) with a 470 μF storage capacitor. This corresponds to a power density near to 6mW/cm3. The experimental AWT energy abilities are presented and the damage detection process is discussed. Finally, some envisaged solutions are introduced for the implementation of the required data processing into an autonomous wireless receiver, in terms of reduction of the energy and memory costs.